1. Technical Field
The present invention relates to a thermal head and a printing device for thermal-transferring a color material on an ink ribbon to a print medium.
2. Related Art
As a printing device for printing images or characters on a print medium, there is a thermal transfer printing device (hereinafter simply referred to as a printing device) which sublimates a color material forming a ink layer provided to one surface of an ink ribbon to thermal-transfer the color material to a print medium, thereby printing color images or characters. The printing device is provided with a thermal head for thermal-transferring the color material on the ink ribbon to the print medium and a platen disposed at a position facing the thermal head and for supporting the ink ribbon and the print medium.
In the printing device, the ink ribbon and the print medium are overlapped so that the ink ribbon faces the thermal head and the print medium faces the platen, and the ink ribbon and the print medium run between the thermal head and the platen while the platen presses the ink ribbon and the print medium against the thermal head. In this case, the printing device applies thermal energy to the ink ribbon running between the thermal head and the platen with the thermal head on the ink layer from the rear face side of the ink ribbon, and sublimates the color material with the thermal energy to thermal-transfer the color material to the print medium, thereby printing color images or characters.
In this thermal transfer printing device, power consumption becomes larger when printing at higher speed because the thermal head needs to be rapidly heated to a high temperature. Therefore, it is difficult particularly in home-use-printing devices to increase printing speeds while achieving lower power consumption. In order for achieving high speed printing particularly by a home-use thermal transfer printing device, it is required to improve the thermal efficiency of the thermal head to reduce power consumption.
As a thermal head for a thermal transfer printing device used from the past, for example, a thermal head 100 shown in FIG. 20 can be cited. The thermal head 100 is composed of a glass layer 102 formed on a ceramic substrate 101, and a heat generating resistor 103, a pair of electrodes 104a, 104b for making the heat generating resistor generate heat, a protective layer 105 for protecting the heat generating resistor 103 and the electrodes 104a, 104b sequentially formed on the glass layer 102. In the thermal head 100, a part of the heat generating resistor 103 exposed from a gap between the pair of electrodes 104a, 104b forms a heat generating section 103a for generating heat. The glass layer 102 is formed to have a substantially circular arc shape in order for making the heat generating section 103a face the ink ribbon and the print medium.
Since the ceramic substrate 101 having high thermal conductivity is used in the thermal head 100, the thermal energy generated from the heat generating section 103a is radiated from the glass layer 102 through the ceramic substrate 101 to rapidly lower the temperature, thus offering a preferable response. However, in the thermal head 100, since the thermal energy in the heat generation section 103a is radiated to the side of the ceramic substrate 101 to easily reduce the temperature, the power consumption in raising the temperature to the sublimation point increases, thus making the thermal efficiency worse. According to the thermal head 100, although the preferable response can be obtained, thermal efficiency is degraded, and accordingly, it is required to heat the heat generating section 103a for a long period of time to obtain a desired depth, which causes large power consumption and makes it difficult to improve the printing speed while achieving low power consumption.
In order for solving such a problem, the inventors of the present invention invented a thermal head 110 as shown in FIG. 21. This thermal head will be explained below as related art of the present invention, in which the thermal head 110 uses a glass layer 111 having lower thermal conductivity than the ceramic substrate instead of the ceramic substrate in order for preventing the thermal energy in thermal-transferring the color material to the print medium from being conducted to the substrate side. The thermal head 110 is composed of a heat generating resistor 112, a pair of electrodes 113a, 113b and protective layer 114 sequentially formed on the glass layer 111 provided with a protruding section 111a having a substantially circular arc shape. The protruding section 111a of the glass layer 111 is formed like a substantially circular arc in order for making a heat generating section 112a of the heat generating resistor 112, which is exposed from a gap between the pair of electrodes 113a, 113b, and generating heat, face the ink ribbon and the print medium.
In the thermal head 110, since the glass layer 111 having lower thermal conductivity than the ceramic substrate 101 shown in FIG. 20 serves as the ceramic substrate 101, it becomes difficult for the thermal energy generated from the heat generating section 112a to be radiated to the side of the glass layer 111. Thus, in the thermal head 110, the quantity of the heat conducted to the ink ribbon side can be increased, thus the temperature thereof can rapidly be raised in thermal-transferring the color material to the print medium. Therefore, it becomes possible to reduce power consumption for raising the temperature to the sublimation temperature, thus making the thermal efficiency more preferable. However, in the thermal head 110, it becomes difficult for the thermal energy stored in the glass layer 111 to be radiated, thus the temperature of the thermal head 110 does not drop immediately because of the thermal energy stored in the glass layer 111, which degrades the response in contrast to the case with the thermal head 100. Thus, in the thermal head 110, since the response is degraded even with the improved thermal efficiency, it is difficult to increase the printing speed.
Since it is required to improve both of the thermal efficiency, which is a downside of the thermal head 100, and the response, which is a downside of the thermal head 110, for achieving high speed printing of high quality images or characters with reduced power consumption in thermal transfer printing devices, the inventors of the present invention further invented a thermal head 120 as shown in FIG. 22. This thermal head will be explained below as further related art of the present invention, in which the thermal head 120 is composed of a heat generating resistor 122, a pair of electrodes 123a, 123b, a protective layer 124 sequentially formed on the glass layer 121 having a protruding section 121a formed like a substantially circular arc in order for making a heat generating section 122a of the heat generating resistor 122, which is exposed from a gap between the pair of electrodes 123a, 123b, face the ink ribbon and the print medium, and inside the glass layer 121, there is formed a groove section 125 filled with air.
In the thermal head 120, by providing a groove section 125 to the glass section 121, the thermal conductivity of the groove section 125 is lowered because of the nature of air of having lower thermal conductivity than glass, thus the heat radiation to the glass layer 121 side can further suppressed than in the case with the thermal head 100 shown in FIG. 20 using the ceramic substrate 101. Thus, in the thermal head 120, the amount of heat conducted to the ink ribbon side increases, and accordingly, the power consumption for raising the temperature to the sublimation temperature of the color material can be reduced when thermal-transferring the color material, thus making the thermal efficiency preferable. Further, in the thermal head 120, since the thickness of the glass layer 121 is made thinner to reduce the heat storage capacity of the glass layer 121 by providing the groove section 125 to the glass layer 121, the thermal energy stored in the glass layer 121 can be radiated in a shorter period of time than in the case with the thermal head 110 shown in FIG. 21 without the groove in the glass layer 111, thus rapidly lowering the temperature when the color material is not thermal-transferred to make the response preferable. According to these facts, in the thermal head 120, both of the thermal efficiency and the response can be made preferable by providing the groove section 125 to the glass layer 121. In other words, the downsides of the thermal head 100 and the thermal head 110 described above can be solved at the same time in the thermal head 120.
However, even in such a thermal head 120, it is required to further improve the thermal efficiency in order for performing high speed printing with further reduced power consumption. Further, in the thermal head 120, the physical strength of the glass layer 121 might be lowered by providing the groove section 125 to the glass layer 121.
The related art is described in JP-A-8-216443.